The peroxisome targeting signal type 1 (PTS1) receptor, Pex5p, of the tetratricopeptide repeat (TPR) motif family is located mostly in the cytosol and mediates the translocation of PTS1 proteins to peroxisomes. As a step towards understanding the mechanisms of protein import into peroxisomes, we investigated the molecular mechanisms involved in PTS1 recognition by Pex5p with regard to requirement of energy and cytosolic factors, using cell-free synthesized acyl-CoA oxidase (AOx) as a PTS1 cargo protein, together with Pex5p and heat-shock protein (Hsp)70 from rat liver. Pex5p was partly associated with peroxisomes of rat liver, was resistant to washing with a high concentration of salt and to alkaline extraction and was inaccessible to protease added externally. Pex5p bound to AOx in an ATP-dependent manner. AOx synthesized in a cell-free translating system from rabbit reticulocyte lysate was imported into peroxisomes without being supplemented with Pex5p and Hsp70, implying that peroxisome-associated Pex5p was released from the membranes and functional in this in vitro import assay. Antibodies against Pex5p and Hsp70 inhibited AOx import. In contrast, AOx synthesized in a wheat-germ lysate required the external addition of Pex5p for import, in which Hsp70 augmented the AOx import. The TPR domain of Pex5p was revealed to bind to the N-terminal part in an Hsp70-independent manner, whereas mutual interaction of the TPR region was noted in the presence of Hsp70. Hsp70 interacted with the TPR domain of Pex5p. Moreover, Hsp70 and ATP synergistically enhanced the binding of Pex5p to the C-terminal PTS1-containing part of AOx, implying that Pex5p recognizes its cargo PTS1 protein by chaperone-assisted as well as energy-dependent mechanisms in vivo.
Abbreviations used: AOx, acyl-CoA oxidase; CHO, Chinese hamster ovary; Cl, Chinese hamster; GST, glutathione S-transferase; Hsp, heat-shock protein; PTS1 and PTS2, peroxisome targeting signal types 1 and 2; TPR, tetratricopeptide repeat.